Of rectification circuits used in a radio apparatus which receives and detects a radio signal and processes a resulting signal, there is one that uses MOSFETs (metal-oxide-semiconductor field-effect transistors). In this rectification circuit, a couple of MOSFETs are connected in series with the gate terminal and the source terminal of each MOSFET connected to each other and with the drain terminal and the source terminal of each MOSFET placed on the positive side and the negative side, respectively. One end of a capacitor is connected to a connection portion between the adjacent MOSFETs, and the other end of the capacitor is connected to a signal input terminal. An AC voltage signal is input to the signal input terminal, and an envelope signal is output as a current signal from the positive side.
The MOSFET has a threshold voltage. A current flows between the source and the drain if the gate-source voltage exceeds the threshold voltage. No current flows between the source and the drain if the gate-source voltage is equal to or lower than the threshold voltage. Then, in order to output a current signal even when a faint AC voltage signal, that is lower than the threshold voltage is input to the signal input terminal, a voltage that is approximately equal to the threshold voltage is applied between the gate and the source. As a result, an apparent threshold voltage becomes 0 V. The input AC voltage signal on a positive side of its waveform exceeds the apparent threshold voltage, and the input AC voltage signal on a negative side of its waveform is equal to or lower than the apparent threshold voltage. Thus, a pulsating current is generated and output from the positive side.
However, where a voltage is applied between the gate and the source of the MOSFET so that the apparent threshold voltage becomes about 0 V, a negative leakage current occurs when the AC voltage signal is on the negative side of its waveform. Since a rectification current is generated based on a difference between positive and negative currents, a signal cannot be detected if the difference becomes too small.